The field of the invention relates to flame-cutting techniques for making windows in casing for drilling laterals in existing wellbores.
FIG. 1a illustrates a traditional method of window forming wherein a starter mill 10 is directed by whipstock 12 into the wall of the casing 14. The typical shape of a window or opening 16 is shown in FIG. 1b. The window is narrow when initially cut and proceeds to its maximum width when the mill 10 is approximately in the position shown in FIG. 1a as its center is in alignment with the wall of the casing 14. The mill 10 tends to bog down at this point because there is little relative rotation when the center of the mill is in alignment with the wall of the casing 14. As a result, typically mills have been designed to have a build-up of cutting material 18 (shown in FIG. 1c) to prevent coring out the starter mill 10 as it bogs down in the position shown in FIG. 1a. Eventually, the starter mill 10 continues to cut and the remainder of the window (shown in FIG. 1b as the lower half) begins to get progressively narrower until the starter mill 10 exits the window completely.
The traditional technique of using one or more mills, even if done in a one-trip technique involving orientation tools coupled with an anchor or packer, is still fairly time-consuming. Accordingly, one of the objectives of the present invention is to dramatically decrease the time required for formation of the window. Additionally, another objective of the invention is to be able to cut any desired window shape in a minimum amount of time, with the result that minimum debris or residue is left after employing the technique. Another objective is to eliminate the typical window profile illustrated in FIG. 1b so that shorter kickoff diverters can be used when drilling the lateral through the window that is produced.
In the past, casing to be cut in two in a wellbore has been cut using techniques involving ignited combustible materials which are directed to a radial nozzle or nozzles to cut cleanly through the casing or tubular. These types of devices using a cutting flame radially to cut through a pipe are illustrated in U.S. Pat. Nos. 4,598,769 and 5,435,394. These devices are limited-purpose tools for cutting radially through a casing.
Explosive techniques for making a window in casing, using a shaped charge attached to a starter mill which is, in turn, attached to a whipstock, is illustrated in U.S. Pat. No. 5,636,692.
Another objective of the present invention is to use flame-cutting techniques to produce a window of desired shape rapidly to facilitate further downhole operations.
These aspects of the method will be more readily understood by those skilled in the art from a review of the details of the preferred embodiment described below.
A tool containing a solid combustible material is used to provide a pipe-cutting flame and to direct a pipe-cutting flame against a casing or tubular. The nozzles can be configured in any desired array and intensity. In the preferred embodiment, a rectangular window is burned away completely if the nozzles are sufficiently intense or the window is formed in segments if other distributions are used. The device can be anchored by a packer or anchor and can be run in the wellbore on tubing, wireline or electric line. The window can be produced in a single trip. The tool can be removed and a kick-off diverter installed adjacent the window for drilling the lateral.